The present invention relates to a thin-film transistor for use as a switching element in, for example, an active matrix liquid crystal display device, and to a liquid crystal display device provided with such a thin-film transistor.
Conventionally, liquid crystal display devices using nematic liquid crystal have been widely used for segment-type liquid crystal display devices in watches, calculators, etc. Recently, the market for such liquid crystal display devices is further expanding as they are applied in word processors, personal computers, car navigation systems, etc. as display means which make the most of features such as thinness, light weight, and low power consumption. Particular attention is being directed to use of nematic liquid crystal in liquid crystal display devices of the active matrix type, in which active elements such as thin-film transistors (TFTs) are used as switching elements for pixels in a matrix arrangement.
In comparison with, for example, CRTs (cathode ray tubes), liquid crystal display devices of this type have advantages such as greatly reduced thickness (depth), ease of performing full color display, and low power consumption, and thus demand for this type of liquid crystal display device is growing in even more fields, such as display devices for laptop and desktop computers, portable and space-saving televisions, display devices for digital cameras and digital video cameras, etc.
An active matrix liquid crystal display device includes an active matrix substrate provided with active matrix circuits using TFTs, a counter substrate opposite the active matrix substrate, provided with a common electrode, and a liquid crystal layer held between the active matrix substrate and the counter substrate. Display operations are performed by controlling voltages applied to the liquid crystal layer.
On the active matrix substrate, a plurality of pixel electrodes are provided in the form of a matrix. Further, on the counter substrate is provided a common electrode opposite the pixel electrodes on the other side of the liquid crystal layer. Voltages are applied to the liquid crystal by the pixel electrodes and the common electrode. Incidentally, the common electrode is generally structured so as to be provided over substantially the entire surface of the counter substrate.
Further, the active matrix substrate is provided with TFTs as active elements, which are switching means for selective driving of the pixel electrodes, each TFT being connected to a pixel electrode. Further, to enable color display, a color filter layer of, for example, red, blue, and green is provided, for example, on the counter substrate or on the active matrix substrate.
In each TFT, a gate electrode is connected to a scanning line, and a source electrode is connected to a grayscale signal line. The ON/OFF state of a given TFT is controlled by inputting a gate signal through the corresponding scanning line, and when the TFT is ON, a data signal is inputted to the pixel electrode through the grayscale signal line.
Each TFT on the active matrix substrate is structured as follows. On a transparent insulating substrate, a gate electrode and a gate signal line (as a scanning line) are provided, and a gate insulating film is provided so as to cover the gate electrode and the remainder of the transparent insulating substrate. On the upper surface of the gate insulating film above the gate electrode is provided a semiconductor layer, the upper surface of which is in turn provided with two n+-Si layers forming the source and drain electrodes. On the upper surfaces of the two n+-Si layers are provided a source signal line (as a grayscale signal line) and a drain extraction electrode, the upper surfaces of which are in turn provided with an interlayer insulating film. The upper surface of the interlayer insulating film is provided with the pixel electrode.
In order to realize large size or high resolution in the liquid crystal display device, it is necessary to provide the gate signal lines and source signal lines of a low-resistance material. Consequently, low-resistance, easily worked metals such as aluminum are widely used as materials for the gate signal lines and source signal lines.
However, with lines made of aluminum, manufacturing steps involving heat processing, performed after formation of the lines, cause hillocks to form in the lines. These hillocks may break through the insulating film, etc. provided above the lines, causing leakage.
As a structure for preventing the formation of these hillocks, Japanese Unexamined Patent Publication Nos. 6-148683/1994 (Tokukaihei 6-148683, published May 27, 1994), 7-128676/1995 (Tokukaihei 7-128676, published May 19, 1995), and 5-158072/1993 (Tokukaihei 5-158072, published Jun. 25, 1993), for example, disclose structures including aluminum lines provided with an upper layer of a metal having a higher melting point than aluminum. Again, Japanese Unexamined Patent Publication No. 6-104437/1994 (Tokukaihei 6-104437, published Apr. 15, 1994) discloses a structure which prevents hillocks by using lines made of aluminum with an anodized surface.
Further, Japanese Unexamined Patent Publication 9-153623/1997 (Tokukaihei 9-153623, published Jun. 10, 1997) discloses a structure which prevents both hillocks and voids by using lines made of an intermediate layer of aluminum between upper and lower layers made of high-melting-point metal films. It is possible in this way to prevent the formation of hillocks by means of a laminar structure of aluminum and a high-melting-point metal, but it is necessary to simplify the manufacturing process in order to minimize increases in the cost of the liquid crystal display device. Accordingly, it is preferable to use high-melting-point materials, such as titanium and molybdenum, which can be patterned at the same time as the aluminum.
Since titanium, in particular, is a material resistant to electric corrosion, a structure for a liquid crystal display device has been proposed whereby gate signal lines are provided in a three-layer structure of an upper layer of titanium, an intermediate layer of aluminum, and a lower layer of titanium. Here, a liquid crystal display device is prepared by providing, on gate signal lines with the foregoing structure, a gate insulating film of silicon nitride formed by plasma CVD (chemical vapor deposition), and then providing a semiconductor layer, source electrodes, and source signal lines.
However, with a structure like the foregoing, there is poor adhesion between the titanium film forming the upper layer of the gate signal line and the gate insulating layer made of a silicon nitride film, and the silicon nitride film may peel off during subsequent manufacturing steps, reducing yield. Furthermore, when the source signal lines are also provided with an equivalent three-layer structure, the same problem occurs between the source signal lines and the interlayer insulating film formed thereon, which is made of a silicon nitride film.
It is an object of the present invention to provide an inexpensive thin-film transistor with stable properties by realizing good adhesion between, for example, a gate signal line and a gate insulating film of silicon nitride provided thereon, and to provide a liquid crystal display device including such a thin-film transistor.
In order to attain the foregoing object, a thin-film transistor according to the present invention may be structured so as to comprise a gate signal line; a gate insulating film made of a silicon nitride film, provided on the gate signal line; a semiconductor layer provided on the gate insulating film; and a source signal line and/or a drain extraction electrode; in which the gate signal line includes a first layer made of a titanium film containing nitrogen, provided in contact with the gate insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer.
In the foregoing structure, the gate signal line includes a first layer made of a titanium film containing nitrogen, provided in contact with the gate insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer. Accordingly, adhesion with the gate insulating film of silicon nitride is improved in comparison with a case in which the layer in contact with the silicon nitride film is made of a titanium film. Consequently, film peeling during the manufacturing process can be suppressed, and a thin-film transistor with more stable properties can be provided with good yield.
Further, a thin-film transistor according to the present invention may be structured so as to comprise a gate signal line; a gate insulating film provided on the gate signal line; a semiconductor layer provided on the gate insulating film; a source signal line and/or a drain extraction electrode; and an interlayer insulating film made of a silicon nitride film, provided on the source signal line and/or drain extraction electrode; in which the source signal line and/or drain extraction line includes a first layer made of a titanium film containing nitrogen, provided in contact with the interlayer insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer.
In the foregoing structure, the source signal line and/or drain extraction line includes a first layer made of a titanium film containing nitrogen, provided in contact with the interlayer insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer. Accordingly, adhesion with the interlayer insulating film of silicon nitride is improved in comparison with a case in which the layer in contact with the silicon nitride film is made of a titanium film. Consequently, film peeling during the manufacturing process can be suppressed, and a thin-film transistor with more stable properties can be provided with good yield.
A liquid crystal display device according to the present invention may be structured so as to comprise an active matrix substrate provided with a plurality of thin-film transistors, a counter substrate provided opposite the active matrix substrate with an interval therebetween, and a liquid crystal layer in the interval between the active matrix substrate and the counter substrate; in which each thin-film transistor comprises a gate signal line; a gate insulating film made of a silicon nitride film, provided on the gate signal line; a semiconductor layer provided on the gate insulating film; and a source signal line and/or a drain extraction electrode; the gate signal line including a first layer made of a titanium film containing nitrogen, provided in contact with the gate insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer.
In the foregoing structure, the gate signal line includes a first layer made of a titanium film containing nitrogen, provided in contact with the gate insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer. Accordingly, adhesion with the gate insulating film of silicon nitride is improved in comparison with a case in which the layer in contact with the silicon nitride film is made of a titanium film. Consequently, film peeling during the manufacturing process can be suppressed, and a thin-film transistor with more stable properties can be provided with good yield.
Further, a liquid crystal display device according to the present invention may be structured so as to comprise an active matrix substrate provided with a plurality of thin-film transistors, a counter substrate provided opposite the active matrix substrate with an interval therebetween, and a liquid crystal layer in the interval between the active matrix substrate and the counter substrate; in which each thin-film transistor comprises a gate signal line; a gate insulating film provided on the gate signal line; a semiconductor layer provided on the gate insulating film; a source signal line and/or a drain extraction electrode; and an interlayer insulating film made of a silicon nitride film, provided on the source signal line and/or drain extraction electrode; the source signal line and/or drain extraction line including a first layer made of a titanium film containing nitrogen, provided in contact with the interlayer insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer.
In the foregoing structure, the source signal line and/or drain extraction line includes a first layer made of a titanium film containing nitrogen, provided in contact with the interlayer insulating film, and a second layer made of a film containing aluminum, provided beneath the first layer. Accordingly, adhesion with the interlayer insulating film of silicon nitride is improved in comparison with a case in which the layer in contact with the silicon nitride film is made of a titanium film. Consequently, film peeling during the manufacturing process can be suppressed, and a thin-film transistor with more stable properties can be provided with good yield.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.